Simian virus 40 (SV40) will be used as a model cellular replicon in the study of both normal and aberrant DNA replication in mammalian cells. This work will focus on perturbations in DNA synthesis due to cellular stressors such as cytotoxic drugs. DNA damage and nucleotide pool imbalance. Studies of abnormal SV40 DNA replication in this laboratory have provided significant new information about both normal and abnormal DNA replication. Since the understanding of each can be improved by a better understanding of the other, certain aspects of normal DNA replication will continue to be a part of these investigations. Early studies in this laboratory with cytotoxic topoisomerase inhibitors have given clues to the nature and location of the swivel enzyme which removes superhelical density generated by moving replication forks, provided convincing evidence that a type II topoisomerase is required to separate newly replicated daughter chromosomes in mammalian cells, and provided evidence for the mechanism of sister chromatid exchange. These lines of investigation will be extended and several other aspects of aberrant DNA replication will be studied during the proposed funding period.